The federal government of the United States has promulgated test methods in 40 CFR Part 60, Appendix A for determining stack gas velocity and volumetric flow rate. If one knows the flow rate and has another monitor which measures the concentration of pollutants in a selected volume of fluid one can calculate the quantity of pollutants emitted over any selected time period. Accordingly, the test methods have been used in various ways, including the verification of the performance of continuous emission monitoring equipment required by other rules.
The United States has additional regulatory requirements in the form of 40 CFR, Parts 72 through 75 (acid rain reduction), which utilize the Appendix A methods. Some recent regulations now require many electric utilities to continuously measure emissions of specified pollutants on a mass per unit time basis. Adoption of these rules has put a new importance on the errors in both the continuous monitor and in the referenced test methods. The new regulations establish monetary value in the form of trading credits to a ton of SO.sub.2 emissions. The value of such emissions is such that for large utilities as much as $1,000,000 per percent error in measured emissions may result.
The methods of Appendix A were introduced into law over 20 years ago. They, in general, use simple laboratory apparatus and manual techniques to make the various measurements. Unfortunately, the methods are error prone and tests under the same conditions often yield different results. There are many sources of error related to the care, speed and experience of the personnel performing the method as well as variability of the test hardware itself. In addition, the method makes compromises for practical reasons which further expand the margin of error. Over the years, the need to reduce the errors in these methods have been the subject of much discussion and little action.
Appendix A of Title 40 of the United States Code of Federal Regulations contains two methods for measuring flow which are used to determine compliance with emission regulations. These methods, known as EPA Methods 1 and 2, have gained prominence because they are used to determine the proper location, as well as to verify the performance of continuous measuring flow monitors. Errors in Method 2 data can be very costly to both the supplier of the monitor and the utility. The supplier is affected because the method can erroneously show the monitor is not meeting the performance guarantee. The utility is affected because the method is used to adjust the continuous monitor. If the method is in error, that error will directly cause an enormous high or low use of the utility's SO.sub.2 allowance and SO.sub.2 trading credits.
In performing the required testing methods the technician is required to make measurements at selected points within the stack. This is done by inserting a tube or probe into the stack through a port in the stack. Method 2 typically uses a type S (also called S-type) pitot tube made to specific dimensions. Method 2 refers to Method 1 to define the points at which the pitot tube must be placed in the stack or duct to be tested. After certain checks, the pitot tube is extended into the stack or duct to the points determined in Method 1. At each point the tester measures the static pressure in the stack and a differential pressure reading. The differential pressure reading is an average of several readings taken at that point. The person doing the test is expected to position the probe so that the pitot tube openings are at precisely the points required by Method 1. In addition, the tester is expected to align the probe with the direction of flow. Unfortunately, each tester is left to his own skill and resourcefulness to accomplish this task.
As the tester proceeds through the test procedure, he is asked to return again and again to the same measurement points with the same alignment of the probe. No position tolerance is provided in the method as an acceptable limit. Unfortunately, the tolerance required to limit the velocity determination to a specific level of error changes as a function of the stack and the type of flow patterns, as well as the skill of the tester.
The tester is often asked to place the pitot tube many feet into the stack or duct. It is not uncommon to see the end of the probe move considerably as a result of the turbulent gas flow. This movement can result in significant error. Such error is conjunctive to error caused by the fact that pressure is averaged and not the square root of the pressure. In addition to all this, the tester is asked to rotate the probe so as to obtain directional information about the flow at each measurement point. No tolerance limit is applied to this procedure.
There is a need for a drive mechanism which can insert the probe into the stack to predetermined positions. When so positioned the probe should not move during the test. The drive mechanism should also enable the tester to return the probe to any test position again and again.
Probes which are used in this environment typically are made of corrosion resistant metal because of the hot, corrosive environment in which they must operate. Frequently, particulates from the stack gas will adhere to the probe. Consequently, any drive system must be able to precisely position particulate encrusted probes.
Finally, the drive mechanism must be durable and not adversely affected by temperature changes and other environmental conditions found in power plants and other monitoring sites.